The present invention provides for oxidatively resistant silicon carbide coated carbon/carbon (C/C) composites which comprise a C/C composite base that is strongly bound to a durable silicon carbide protective layer, a method for the preparation of these materials using a reactive carbon powder as a reactant in the step of forming the silicon carbide layer, and the use of the silicon carbide coated C/C composite in high temperature applications, preferably in brakes for airplanes.
When the C/C composites are utilized as a stack of discs in airplane brakes, they are required to absorb large amounts of kinetic energy in order to stop the aircraft during landing or in the event of a rejected take-off. During some of the stops, the carbon is heated to sufficiently high temperatures that surfaces exposed to air will oxidize. Some conventional carbon composites have the necessary thermal and mechanical properties required for specific brake designs; however, these conventional composites have open porosities (typically 5% to 10%) which permit internal oxidation. The internal oxidation weakens the material in and around the brake rotor lugs or stator slots, which are areas that transmit the torque during braking.
Damage associated with oxidation has led to premature removal of carbon brake discs on a variety of aircraft, from all current brake manufacturers. Potassium or sodium has, at times, been identified in the severely oxidized regions, and alkali (e.g. sodium and potassium) and alkaline earth elements are well known to catalyze carbon oxidation. Catalyzed oxidation is carbon oxidation that is accelerated by the presence of contaminating materials. These contaminating materials come into contact with the brake from cleaning and de-icing chemicals used on aircraft, and, in particular, from de-icers used on airport runways. These liquids, and other deicers or cleaners containing K or Na, can penetrate the porous carbon discs leaving catalytic deposits within the pores. When such contamination occurs, the rate of carbon loss by oxidation can be increased by as much as two orders of magnitude. The ability of these materials to catalyze oxidation in brake materials has been verified in the laboratory.
It is a problem within this field of technology to protect C/C composites at elevated temperatures up to and exceeding 850xc2x0 C., and to significantly reduce catalytic oxidation at normal operating temperatures. Both field data and theoretical models indicate that modern C/C aircraft brakes frequently see peak temperatures above 850xc2x0 C. and that some models may experience extended periods between 800xc2x0 C. to 1200xc2x0 C. over their service lives.
A known method to improve oxidation resistance is by coating the non-friction surfaces of the composite with materials which act as oxidation inhibitors and seal the surface to limit oxygen access.
When a protective layer of silicon carbide is applied directly onto the C/C composite base, the silicon carbide is highly flaw sensitive. Breach of the silicon carbide layer may occur during the curing step when pressure and/or temperature changes are inadvertently performed too rapidly. Rapid thermal transients induced during component use can also be a major cause of cracking due to the thermal expansion difference between carbon and silicon carbide. Accordingly, what is desired is a method of forming a silicon carbide protective layer which is strongly bound to a C/C composite base wherein the silicon carbide layer is formed on a relatively flaw free smooth coherent surface.
Kaplan et al. (U.S. Pat. No. 5,283,109) teach a silicon carbide coated carbon composite formed with a carbon interlayer. The carbon interlayer is prepared by coating a carbon composite base with a paste-like mixture of carbon powder and a liquid carrier followed by curing. The coated carbon composite is then subjected to chemical vapor deposition with silicon carbide. Thus an open porous layer is needed to allow penetration of the chemical vapor. This final coated composite is inadequate for applications such as airplane brakes, due to a tendency for the composite to crack or peel under extreme conditions as a result of the relatively weak bond between the carbon interlayer and either the carbon composite base or the silicon carbide layer.
The present invention provides an oxidatively resistant C/C composite which is stable under extreme conditions.
The present invention, in part, is a recognition that further treating the SiC coated C/C composite with a phosphoric acid-based retardant solution significantly improves the oxidative resistance at the high end of the typical operating temperature range and in the presence of high concentrations of known oxidation catalysts, such as potassium acetate, a common constituent in aircraft runway deicers. The improvement to the oxidative resistance is unexpected in view of the apparent synergistic interaction between the SiC coating and the phosphoric acid-based retardant solution.
In particular, the present invention, in part, provides for a silicon carbide coated C/C composite, which is resistant to oxidation at high temperatures comprising:
(a) a base formed of a C/C composite,
(b) a layer of silicon carbide formed on a surface of the base (a), wherein the silicon carbide layer (b) is firmly bonded to the C/C composite base (a) through wicked silicon. The silicon carbide coated C/C composite may be further treated with a retardant solution which comprises the ions formed from the combination of the following: 10-80 wt % H2O and 90-20 wt % H3PO4.
For purposes of this disclosure, xe2x80x9cwickingxe2x80x9d is defined as the tendency for a liquid to travel along a fiber upon contact due to the affinity between the fiber and the liquid.
The present invention also provides, in part, a method for the preparation of these materials using a reactive carbon powder as a reactant in the step of forming the silicon carbide layer, and a method for the use of the silicon carbide coated C/C composite in high temperature applications.
The oxidatively resistant C/C composites according to the present invention are preferably used in brakes for airplanes, but may also be used in other high temperature applications, such as electrodes for arc melting of steel, mold stock for metal casting, rocket nozzles, furnace linings, and Hall cell anodes.